We provide locality data for stream populations of Procambarus clarkii (Red Swamp Crawfish) in Maryland. This non-native species is now established in 14 watersheds in the Coastal Plain, including all watersheds where it was historically raised in aquaculture ponds. Our surveys indicate that the introduction of Red Swamp Crawfish in Maryland has largely resulted from aquaculture, although the aquarium, biological supply, and live-bait industries are other potential vectors. The effects of Red Swamp Crawfish on the composition and diversity of stream fauna and flora in Maryland are unknown. The establishment of this species may have negative effects on native crayfishes, especially the congeneric P. acutus (White River Crawfish). These locality records provide baseline information for future monitoring of this non-native species and assessment of its effects on Maryland's stream ecosystems and native crayfishes.